1. Field of the Invention
The present invention relates to a method of dry etching a silicon oxide film or a multilayer oxide film thereof formed on a single-crystal or polycrystal silicon and, more particularly, to the selection of etching gases for etching a silicon oxide film on a silicon substrate with a gas plasma.
2. Description of the Prior Art
Recently, with the progress in the micronization of semiconductor devices, the etching process in the manufacturing of a semiconductor device tends to change from the conventional wet etching techniques utilizing chemical solution to dry etching techniques in which gases in plasma condition or ion beams are utilized. The latter dry etching technique has an advantage in that it involves little possibility of creating a pollution problem due to waste water disposal and makes micropattern processing possible. In addition, it makes it possible to apply a uniform etching treatment. The technique is now indispensable to the manufacture of VLSI in particular.
For purposes of dry etching a silicon oxide film or silicon nitride film formed on a silicon substrate, the following gas systems are employed as etching process gases for dry etching methods hitherto developed.
(A) a mixed gas of CHF.sub.3 with O.sub.2 ; PA0 (B) a mixed gas of CHF.sub.3 with other fluorocarbon gas, such as CF.sub.4 or C.sub.2 F.sub.6 etc.; PA0 (C) a mixed gas of C.sub.4 F.sub.8 with CH.sub.2 F.sub.2 or CH.sub.3 F.
In order to dry etch the oxide film to good dimensional precision, it is necessary to form a deposit on the etching side wall to protect the etching side wall from ions and/or radicals in a plasma.
FIG. 4 is a sectional view of a process to form a contact window 6 by the etching treatment of using a conventional dry etching method, said process being an example of a method using a mixed gas mentioned in (A) above, using a diode parallel plates RIE system and under the conditions of CHF.sub.3 :O.sub.2 :He=9:1:40; a gas pressure of 100 m Torr; and RF power of 330 W (13.56 MHz). First, as shown in FIG. 4(a), a CVD oxide film 21 of 0.1 .mu.m in thickness, a BPSG film 22 of 0.4 .mu.m in thickness, a CVD oxide film 21 of 0.25 .mu.m in thickness, and a BPSG film 22 of 0.45 .mu.m in thickness are sequentially deposited on a single crystal silicon substrate 1 to form multilayer oxide films 2 having a total thickness of 1.2 .mu.m, and then a resist 3 which serves as an etching mask is formed with an opening pattern 4 of 0.65 .mu.m square on the multilayer oxide films 2. Next, as shown in FIG. 4(b), the multilayer oxide films 2 are etched using the resist 3 as a mask under the foregoing conditions. In this case, a deposit 5 is formed on the etching side wall. Further, as shown in FIG. 4(c), the resist 3 and the deposit 5 are removed to form the contact hole 6.
FIG. 5 is a sectional view of a process to form a contact hole 6 by an etching treatment using a conventional dry etching method, said process being an example of a method to use a mixed gas mentioned in (B) above, using a diode parallel plates RIE system, showing by way of example under the conditions of CHF.sub.3 :C.sub.2 F.sub.6 :He=9:1:1 (where, the number of H element atoms contained in the etching process gases is 0.82 times the number of C element atoms); a gas pressure of 100 m Torr; and RF power of 330 W (13.56 MHz). First, as shown in FIG. 5(a), a CVD oxide film 21 of 0.1 .mu.m in thickness, a BPSG film 22 of 0.4 .mu.m in thickness, a CVD oxide film 21 of 0.25 .mu.m in thickness, and a BPSG film 22 of 0.45 .mu.m in thickness are sequentially deposited on a single crystal silicon substrate 1 to form multilayer oxide films 2 having a total thickness of 1.2 .mu.m, and then a resist 3 which serves as an etching mask is formed with an opening pattern 4 of 0.65 .mu.m square on the multilayer oxide films 2. Next, as shown in FIG. 5(b), the multilayer oxide films 2 are etched using the resist 3 as a mask under the foregoing conditions. Further, as shown in FIG. 5(c), the resist 3 and the deposit 5 are removed to form the contact hole 6.
Similarly, FIG. 6 is also a sectional view of a process to form a contact hole 6 by etching treatment using a conventional dry etching method, said process being an example of a method to use a mixed gas mentioned in (B) above using a diode parallel plates RIE system, under the conditions of CHF.sub.3 :CF.sub.4 :He=1:1:5 (where, the number of H element atoms contained in the etching process gases is 0.5 times the number of C element atoms); a gas pressure of 100 m Torr; and RF power of 330 W (13.56 MHz). First, as shown in FIG. 6(a), a CVD oxide film 21 of 0.1 .mu.m in thickness, a BPSG film 22 of 0.4 .mu.m in thickness, a CVD oxide film 21 of 0.25 .mu.m in thickness, and a BPSG film 22 of 0.45 .mu.m in thickness are sequentially deposited on a single crystal silicon substrate 1 to form multi-layer oxide films 2 having a total thickness of 1.2 .mu.m, and then resist 3 which serves as an etching mask is formed with an opening pattern 4 of 0.65 .mu.m square on the multilayer oxide films 2. Next, as shown in FIG. 6(b), the multilayer oxide films 2 are etched using the resist 3 as a mask under the foregoing conditions. Further, as shown in FIG. 5(c), the resist 3 and the deposit 5 are removed to form the contact hole 6.
FIG. 7(b) shows the ratio of minimum thickness to maximum thickness, Tmin/Tmax (hereinafter referred to as step coverage), of a deposit 5 produced by plasma reaction as deposited on a vertically stepped surface as shown in FIG. 7(a), which ratio differs according to the kind of gas used as shown.
In the above described conventional dry etching method, in the case of the mixed gas shown in (A) above, a large amount of O radicals is produced to react with the resist 3 and, therefore, as shown in FIG. 4(b), the end of the resist 3 retreats from the dotted line to the solid line. The deposit 5 which has been deposited to protect the side wall of the contact hole 6 also diminishes as it reacts with O radicals. As a consequence, the contact window 6 is dimensionally enlarged to the extent that, as shown in FIGS. 4(a) and 4(c), the contact window 6 is widened from the resist opening pattern 4 of 0.65 .mu.m square to a size of 0.85 .mu.m square, which means a considerable decrease in pattern transfer precision.
In the case where the mixed gas shown in (B) above is used, because the CHF.sub.3 is used therein, and the step coverage is as small as 5% as shown in FIG. 7(b), therefore, the deposit 5 attached to the side wall of the contact hole is considerably smaller despite the fact that the number of H element atoms contained in the etching process gases is 0.82 times the number of C element atoms. Consequently, as is the case with the mixed gas mentioned in (A) above, the effect of the deposit for protecting the side wall of the contact hole 6 is reduced, so that the size A of the contact hole is enlarged; and thus, as shown in FIG. 5(a) and 5(c), the contact window 6 formed on the basis of the resist opening pattern 4 of 0.65 .mu.m square is widened to 0.9 .mu.m square, which also means a considerable decrease in pattern transfer accuracy.
Furthermore, even when the quantity of the deposit on the etching side wall is increased using the mixed gas of (B) above, with changed proportions of ingredients therein, the step coverage of the deposit 5 produced by plasma reaction as shown in FIG. 5 using CHF.sub.3 is as small as 5% and, therefore, as shown in FIG. 6(b) minute configurational changes of the etching side wall which occur as a consequence of the etching rate difference due to the film species of multilayer oxide films 2 are often reflected as a film thickness irregularity of the deposit 5 attached to the etching side wall. Further, said deposit 5 is composed of slightly soluble materials such as (C.sub.x F.sub.z)n polymer, which have a high percentage content of F element, and, therefore, is hard to remove in the washing step of an after-processing. Thus, as shown in FIG. 6(c), a deposit 5 as an etching residue is present on the side wall of the contact hole.
In the case of the mixed gas shown in (C) above, while the step coverage of CH.sub.2 F.sub.2 or CH.sub.3 F, as an additive gas for configuration control is good, the step coverage of C.sub.4 F.sub.8 of principal gas is poor, which eventually poses a problem similar to that in the case of the mixed gas (B).
As a result of the investigation of the present inventors, it has been found that the foregoing method involves the following problems. When such a gas that the step coverage of the deposit produced by plasma reaction is low in the order of 5% is used, the amount of the deposit 5 attached to the etching side wall is small despite the fact that the number of H element atoms contained in the etching process gases is 0.65 or less times the number of C element atoms. That is to say, etching is carried out under conditions which would result in greater polymer production. As a consequence, the effect of protection of the etching side wall is reduced and, therefore, the etching size is extended. It being thus impossible to form such dimensionally accurate contact holes as shown in FIGS. 2 and 3 in the process of microfine working at submicron level through dry etching.
Next, even when the amount of deposit 5 attached to the etching side wall is increased by changing the proportions of component gases in the mixed gas, where such a gas that the step coverage of the deposit produced by plasma reaction is low is used, minute configurational changes on the etching side wall which result from the difference on etching rate between the different component films of the multilayer oxide films 2 are largely reflected as interlayer variations in the deposit 5 attached to the etching side wall. Further, since the deposit 5 is a less soluble product composed of a material having a high F element content, e.g., (C.sub.x F.sub.z)n polymer, the deposit is hard to remove in the washing step of an after-etching stage. Therefore, as shown in FIG. 6(c), a deposit 5 as an etching residue is present on the side wall of the contact hole, with the result that the stability of the etching configuration is unfavorably affected.
This invention has been developed in view of the foregoing drawbacks of the prior art, and accordingly it is a primary objective of the invention to provide a dry etching method which enables dimensionally accurate formation of contact windows in the process of superfine working at submicron level and assures good stability of etching configuration.